This invention relates to a method and apparatus for sampling, quantizing and processing a video signal, especially a method and apparatus for minimizing deterioration of picture quality accompanied with quantization.
There are many types of known apparatus for processing a video signal. For example, an apparatus for converting the video signal into a digital signal; an apparatus for memorizing the video signal of one field or one frame and processing the same; an apparatus for filtering the video signal or compensating the signal by use of a line memory; an apparatus for eliminating a time base fluctuation of the video signal by a buffer memory; an apparatus for time-compressing the video signal and transmitting the signal in a time-division multiplex system; an apparatus for band-compressing a highly defined video signal and transmitting the signal in the time-division multiplex system; etc. In all such known apparatus, it is necessary to sample and quantize the video signal by using sampling clock pulses with sequential processing at each sample value.
In order to produce the sampling clock pulses, the most simple known method utilizes asynchronous and independent pulses. However, such method causes some problems in the aforementioned apparatus. For example, in the apparatus using the buffer memory, time base fluctuations generate in a processed video signal according to quantizing errors on the time base due to asynchronism between the video signal and the sampling clock pulses, and in an apparatus for time-compressing and expanding the video signal, the quantizing errors are magnified so that the quality of a picture reproduced from the processed video signal is degraded. Further, since the video signal is a broad band signal, it is necessary to provide a sufficiently high sampling clock pulse frequency, but such frequency cannot be obtained due to a limit of the working speed of the apparatus. Therefore, the sampling clock frequency approaches the video signal band so that spurious components are generated. As the spurious components appear as moire noise in a reproduced picture, it is necessary to restrict the video signal band sufficiently and rapidly before processing the video signal. However, the rapid restriction of the band gives rise to a delay distortion and as a result, a waveform distortion, for example, a ringing is generated.
Another method, which uses an automatic frequency controlled circuit (hereinafter AFC circuit) as shown in FIG. 1, is well known for producing the sampling clock pulses. Referring to FIG. 1, the numerals 1 and 2 denote an input terminal of the video signal and an output terminal of the sampling clock pulses, respectively. A horizontal synchronizing signal is separated from the input video signal by a horizontal synchronous signal separator 3 and supplied to one input terminal of a phase comparator 4. The output of the phase comparator is supplied to a voltage controlled oscillator 6 (hereinafter VCO) via a phase compensator 5. An output signal of the VCO is divided by a divider 7 which produces an output signal having the same frequency as a horizontal scanning frequency of the input video signal for phase comparison with the horizontal synchronizing signal by the phase comparator 4. An error voltage output signal is supplied in response to a phase difference of the both signals to the VCO 6 as a control voltage therefor. The AFC circuit, which is constructed as described above, supplies the sampling clock pulses synchronized with the input video signal from the output terminal 2. As this prior method is based upon a feedback control, it also presents some problems. For example, a phase fluctuation of the sampling clock pulses remains due to circuit disturbance. If the dividing ratio of the divider 7 becomes large, a phase distance between the sampling clock pulses and the horizontal synchronizing signal becomes large, so that the remainder of the phase fluctuation also becomes large. Further, if the input video signal has a time base fluctuation, the AFC circuit generates a follow-up error. If the response speed of the AFC circuit is increased in order to enhance the follow-up ability thereof, the AFC circuit then also has a high response to noise contained in the input video signal. As a result, the AFC circuit becomes unstable. Also, if the time base fluctuation of the input video signal becomes large, the AFC circuit deviates from the synchronous range, so that it becomes inoperative.